1. Field of the Invention
This invention relates generally to a method and apparatus for controlling the operation of a gas discharge tube and, more particularly, to a method and apparatus for digitally controlling the length along the tube that is illuminated and the light intensity of a cold cathode gas discharge tube using a pulse-width modluated signal.
2. Description of the Background
Lighting devices that provide illumination by means other than incandescence are well-known and extremely common. Among such lighting devices are ionizable gas tubes that contain neon, argon and the like which are generally known as gas discharge tubes. In these devices the light is produced on the basis of excitation of an ionizable gas contained within a tube.
A large number of different approaches to exciting the gas in the tube are known, however, all have a common principle of applying a high, alternating current (AC) potential between two electrodes of the tube, or a variation in which the high voltage AC is applied to one electrode of the tube, with the other electrode being connected to ground. This AC high voltage causes a small current to flow through the gas inside the tube, thereby ionizing the gas and producing a plasma that ultimately releases energy in the form of visible light.
So-called fluorescent lights are fired or operated by maintaining a cathode heater voltage to provide electron emission, thereby helping ionization of the gas. A variation of such approach is the so-called cold cathode gas discharge tube, which can be started/operated using circuitry that provides a high current at the starting time and then drops the current across a ballast during operation.
Gas discharge tubes are generally thought to provide the best illumination when excited by a high voltage in the range of 5-10 kilovolts, which alternates at a frequency of at least 20 KHz. Typically, the necessary high-voltage potential is provided by a free running oscillator operating at around 20 KHz, which is then connected to the primary side of a high-voltage transformer.
There have been previously proposed various approaches to controlling the length of illumination of the tube and/or the brightness of illumination of the tube, all of which generally involve an analog circuit that controls the current flowing through the primary winding of a high-voltage transformer. These circuits have proven to be less than acceptable, both operationally and commercially, because of their complete dependence on component tolerances that are adversely affected by degradation and aging, thereby providing poor repeatability and instability of the circuitry, as well as involving an inherently high power consumption.
As noted, it is known that a relationship exists between the length of illumination along the tube and brightness of the illuminated tube relative to the potential applied to such gas-filled tube. For example, as the electrostatic potential is increased across the gas tube, a certain voltage level is reached at which the gas starts to ionize. At that instant, only a small portion of the tube in the vicinity of the high-voltage electrode will be illuminated. As the voltage is further increased, more and more of the length of the tube is progressively illuminated until the plasma, caused by the ionized gas in the tube, reaches the opposite end of the tube at which the other electrode is arranged. At that time, the entire length of the tube is illuminated and additional increases in voltage will produce corresponding increases in the light output intensity.
A variation on this principle is disclosed in U.S. Pat. No. 4,742,278 in which a cold cathode gas discharge tube includes a power source connected to only a single cathode element. The power source provides an AC voltage referenced to ground having a high enough frequency to ionize the gas through the natural surrounding capacitance between the ionized gas and the ground potential. The power source output is a ramp voltage that increases to move the ionization point along the tube from one end to the other. By controlling the ramp level, the ionization point along the length of the tube and, thus, the length of illumination can be controlled and special lighting effects can be achieved. A retrace or flyback period is provided after each ramp during which the voltage drops to zero and then commences the ramp again. This above-identified patent further discloses that a display driver unit could be responsive to various external functions, such as an audio signal, to create different display patterns using cold-cathode, gas discharge tubes. The system described in the above identified patent suffers from the same drawbacks as described above in that the system is highly dependent upon tolerances and degradation/aging of regulation elements, such as potentiometers, capacitors and the like, resulting in poor reliability, instability, and high power dissipation.